This application claims priority under 35 U.S.C. xc2xa7 119 to Japanese Patent Application No. 2000-028222 filed on Feb. 04, 2000, the entire contents of which are hereby incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to an image processing apparatus, such as a digital copier, a facsimile, etc. that quantizes an input image signal into multilevel data, and in particular relates an image processing apparatus capable of outputting an image of one pixel at a multilevel.
2. Discussion of the Background
In general, as a pseudo halftone processing technique in which a halftone is reproduced using binary dots (i.e., dot existence/no existence), an error defusing method has been known. According to such an error defusing method, an error arising when each pixel is made into binary data is defused to a plurality of circumjacent pixels. Also, a multilevel error defusing method has been known as an extension of the error defusing method to a multilevel. The multilevel error defusing method defuses an error, which arises when a number of input levels of each pixel is converted into a fewer number of output levels, to a plurality of circumjacent pixels.
As one example of this type of background quantizing technique, Japanese Patent No. 2581662 proposes a method in which a performance of a halftone image is based on sharpness of character. The method recognizes characteristics of an image from an image signal of a pixel in the vicinity of a target pixel and outputs an image with a decreased number of multilevels in a character section and an increased number of multilevels in a halftone image section, by switching a number of multilevel quantization in the multilevel error diffusion. The characteristics of an image are typically related to an average of multilevel data of circumjacent of a target pixel that has already been quantized, and an edge value of the image.
Such a background technique indicates that the quality of an image (e.g. a picture pattern section) is fine if a number of output levels is larger because a performance is improved.
However, depending on development and charge performances of an image forming engine and driving precision of a roller, a photosensitive member, etc., a stable image reproduction can occasionally be obtained when a number of output levels is reduced rather than increased.
Further, when an image forming engine having relatively linear development characteristics in relation to an exposure amount, as illustrated in FIG. 18, is compared with one having development characteristics of sharp increase at more than a prescribed exposure level, as illustrated in FIG. 19, density reproduction performance in an output image of the error diffusing method whose number of output levels is large is inferior when utilized in a device having the development characteristics of FIG. 19. This is because stability is inferior to the characteristics of FIG. 18 due to multilevel exposure executed at a sharp inclination section of the development characteristics.
In addition, when a writing operation is executed at a middle level, an image is generally affected greatly by uneven charge and drive, and as a result, a so called banding phenomenon (i.e., a band formed in an image in a sheet feeding direction) tends to appear in a middle density section, and thereby image quality sometimes is inferior. Thus, in such a situation, image quality is fine if a writing operation is performed using a binary error diffusion technique not at a multilevel.
However, a problem may arise with a dropout appearing in a high density image range when processing with the binary error diffusion technique. Specifically, since a binary error diffusion process uses a so called area gradation technique that represents density with a black pixel and a white pixel, a white dropout dot exists in a solid black in a high density range in which density is slightly lower than a solid black section and visually gives an ill affect.
For such representation of gradation of a high density section, a multilevel error diffusion technique is more superior to a binary error diffusion technique. Namely, according to the multilevel error diffusion technique, such an image is reproduced such that a thin dot can be formed in a rigid black and dropout does not appear so as not to visually give an ill affect. FIG. 20 illustrates a relation between the above-discussed properties of the binary error and multivalue error diffusion techniques, and as noted therefrom, the binary error diffusion technique is preferable with regard to an anti-banding performance in low to middle density sections, but dropout is noticeable in a rigid black section. To the contrary, the multilevel error diffusion technique is preferable with regard to the dropout in the rigid black section, but banding is noticeable in low to middle density sections.
Accordingly, an object of the present invention is to address and resolve the above and other problems and to provide a novel image processing apparatus.
The above and other objects are achieved according to the present invention by providing a novel image processing apparatus that converts input image data having a first multivalue to image data having a second multivalue which is less than the first multivalue by using a multivalue error diffusion technique. The image processing apparatus includes a threshold generating device configured to generate a plurality of variable thresholds at least based on a characteristic of a target pixel data, wherein the variable thresholds have a number less than the second multivalue by one. A quantization device is provided to quantize the input image data of the target pixel into the image data having the second multivalue based on the variable thresholds. The variable thresholds are varied in a manner such that prescribed quantized data quantized by the quantization device does not create noticeable banding in an image substantially in a middle density range and does not create noticeable dropout in an image substantially in a high density range.
In another embodiment, the threshold generating device generates the variable thresholds based on input image data of the target pixel and at least one circumjacent pixel.
In yet another embodiment, the variable thresholds are determined based on an average of the input image data of the target pixel and the at least one circumjacent pixel.
In yet another embodiment, a difference of the variable thresholds becomes large in accordance with an increase in a value of the input image data of the target pixel.
In yet another embodiment, the variable thresholds are substantially the same even if the input image data of the target pixel increases in value.
In yet another embodiment, the variable thresholds are substantially entirely or partially equal to each other, when a value of input image data of the target pixel is less than a prescribed level.
In yet another embodiment, the lowermost variable thresholds are fixed to a prescribed level.
In yet another embodiment, the threshold generating device selects and sets one set of variable thresholds from a plurality of thresholds sets.